A variety of gas-barrier members have heretofore been proposed and, particularly, a polyvinylidene chloride, a polyacrylonitrile and an ethylene/vinyl alcohol copolymer have been known as gas-barrier resins. From the environmental problems, however, it is a tendency to refrain from using the polyvinylidene chloride and the polyacrylonitrile. The ethylene/vinyl alcohol copolymer, on the other hand, permits the gas-barrier property to vary much depending upon the humidity; i.e., the gas-barrier property decreases under highly humid conditions.
In order to solve the above problems, there have been proposed a gas-barrier resin composition in which a polycarboxylic acid polymer is reacted with a crosslinking agent having 2 to 4 functional groups that react with the carboxyl groups and with metal ions having a valency of two or more so that the polycarboxylic acid polymer forms crosslinked portions due to the crosslinking agent and, further, forms the crosslinked portions due to the metal ions having the valency or two or more, the weight ratio of the polycarboxylic acid polymer and the crosslinking agent being 99.9/0.1 to 65/35 (patent document 1), and a gas-barrier film having a gas-barrier coating formed on at least one surface of a thermoplastic resin film, the coating being formed from a polyacrylic acid crosslinked with a crosslinking agent which contains an epoxy compound having not less than 3 epoxy groups per a molecule, and the crosslinking agent being contained in an amount of 1 to 100 parts by mass per 100 parts by mass of the polyacrylic acid (patent document 2).
The gas-barrier materials disclosed in the above patent documents 1 and 2 must be crosslinked to a high degree through the heating at a high temperature of not lower than 150° C. or conducted for a long period of time and, therefore, greatly affect the plastic substrate and, besides, require the dipping treatment or the spray treatment for conducting the ionic crosslinking with metal ions, deteriorating productivity and consuming large amounts of energy and water. Besides, these gas-barrier materials fail to satisfy the requirements of flexibility and retort resistance.
There has, further, been proposed a gas-barrier laminated film which enables dry-firing to be effected at a relatively low temperature, the gas-barrier laminated film comprising a layer (a) formed from a coating solution (A) containing a polycarboxylic acid polymer without heat treatment and a layer (b) formed from a coating solution (B) containing a water-soluble multivalent metal salt and an aqueous resin, the layer (a) formed from the coating solution (A) and the layer (b) formed from the coating solution (B) forming at least a pair of lamination units neighboring each other (patent document 3).
Patent document 1: JP-A-2003-171419
Patent document 2: JP-A-2002-240207
Patent document 3: JP-A-2007-313758